"Had I been a terrorist I could have assassinated him at any moment," he wrote in Friday's newspaper.

During his time in employment he says he got close to ministers, MPs, police officers and VIPs.

On Thursday he carried the components of a fake bomb into the building in a black shoulder bag, via the underground station entrance.

He was carrying 200g of modelling clay - which resembles the explosive semtex - batteries and set of wires. An alarm clock 'timer' was in his jacket pocket.

The materials Mr France were carrying "couldn't have been anything other" than a bomb, the only difference being he had modelling clay instead of semtex, said Mr Dudman.

Mr France was waved through at reception by a police officer, he says.

He video recorded himself assembling the "bomb" in toilets near the MPs' restaurant.

Mr Dudman told Today: "It just simply beggars belief that on the day that ... everybody was saying: 'I can assure you we are going to ratchet up security, it's never going to be tighter', that same day a man from the Sun walks in carrying what could have been a bomb.